May
21
Filed Under (CDOM, gliders, OMZ) by Amanda Whitmire on 21-05-2009

I spent some time today looking at patterns of CDOM along our glider sections. As an introduction, CDOM is colored dissolved organic matter, also known as “yellow substance,” “gilvin,” and “gelbstoff.” CDOM is a subset of the pool of dissolved organic material in the ocean (and lakes, streams, estuaries, etc.) that is optically active (i.e. has color). CDOM appears yellow or brown to the eye depending on it’s concentration, it absorbs light very strongly in the blue region of the spectrum, and fluoresces in the blue as well. CDOMs optical characteristics enable us to monitor it’s concentration and distribution with a fluorometer on the Seaglider (WET Labs ECO-Puck; CDOM fluorescence excitation/emission at 370/460 nm). CDOM is an important parameter to keep track of for many reasons (see excellent review by P. Coble, Chem. Rev., 2007, 107, 402-418), but for our purposes we are primarily interested in monitoring the variability of sources and sinks of carbon in the OMZ.

Here is a plot of all of the CDOM-depth profiles from our most recent complete section, Line 11, with oxygen concentration in color. The red line is a running average at each depth bin.

CDOM profiles from SG157, Line 11

CDOM profiles from SG157, Line 11

A couple of things pop out right away. CDOM is degraded by sunlight very quickly, which is evident here in the surface data. That’s just a good double-check that the fluorometer is working. Second, there appears to be (maybe?) two discrete pools of CDOM – one associted with phytoplankton production and degredation in the chlorophyll maximum (photic zone, high O2), and one associated with the microbial community in the OMZ (low light/aphotic & hypoxic). However, it is impossible to tell from these data alone if the source of the CDOM in the OMZ is local or if it was derived from far away sources and has been transported with the water mass. Collecting water samples for CDOM spectral absorption¬† measurements will tell us a great deal about the nature and origin of CDOM in this area. It’s also important to remeber that not all CDOM is fluorescent, so we are actually looking at a sub-pool (the fluorescent bits) of a sub-pool (the colored bits) of the pool of dissolved carbon. However, the CDOM data that we are able to collect autonomously could be very instructive when considered in conjuncion with other variables like chlorophyll, backscattering, and physical indecies of mixing and mass transport.

This is just a first-look at the data, and I’m still trying to get my head around it. Feedback in the comments is encouraged!